To deal rationally with the complexity of present communication systems and with the need to make different systems mutually compatible, the International Standards Organization ("ISO") developed a model for specifying such systems. Using this model, called the Open Systems Interconnect ("OSI") model, a communication system can be broken down into a hierarchial structure that permits standards to be defined at each level in the structure. The OSI model provides a hierarchy of seven different layers. that can occur in a communication system. Each layer in the OSI model covers a different function performed by the communication system.
The lowest layer in the OSI model, called the physical layer, specifies the physical structure of interfaces in a particular communication system or network. Thus, a standard for the physical layer of a communication system specifies such things as the number of wires, their electrical characteristics, the characteristics of signals transmitted over the wires, connectors used for joining two sets of wires into a single longer set of wires, etc.
The next higher layer in the OSI model, called the data link layer, specifies how data is transmitted error free through the communication system. Thus, a standard for the second layer in the OSI model specifies how to detect errors in transmissions passing over the physical layer, and how to correct any errors that may occur during transmission.
The next higher layer in the OSI model, called the network layer, specifies the manner in which connections are formed between various places in the communication system for transmitting data between them. The standard for the third layer in the OSI model, therefore, specifies the signals transmitted over the data link layer that cause the communication system to transfer data between two places on the network.
A standard defined by an International Telegraph and Telephone Consultative Committee ("CCITT") for the ISDN communication channel specifies these three lowest levels in the OSI model. Under the CCITT standard, a basic ISDN access consists of two full-duplex 64 kilobits per second ("kbps") digital data channels, called channel B1 and channel B2, plus another full-duplex 16-kbps digital channel, called a D channel. Under the CCITT standard, using time division multiplexing, all three of these digital data channels may be transmitted over a single pair of twisted wires, or over two pairs of twisted wires. ISDN basic access, as specified by CCITT, was originally intended to provide a basic digital data transmission capability suitable for use by individuals such as in their homes or small businesses.
When ISDN basic access was initially specified, each of the B channels was intended to carry either:
1. digital data, such as that from a personal computer or from a computer terminal; PA1 2. Pulse Code Modulation ("PCM") encoded digital voice communication; or PA1 3. a mixture of lower data rate communications including digital data and digitized voice that were each encoded at a fraction of each B channel's full 64-kbps capacity.
Under the ISDN specification, the D channel serves two purposes. First, the D channel carries signaling information that controls the transmission of data over the two B channels. In addition, when the D channel is not carrying signaling information, it may be used to transmit packet-switching or low-speed telemetry. The combined data rate at which digital data may be transmitted over twisted pairs of wires in accordance with the ISDN standard for basic access is 144-kbps, i.e. 128-kbps for the combined B1 and B2 channels plus 16-kbps for the D channel.
In addition to the ISDN basic access specified by CCITT, that organization has also specified a higher performance ISDN communication channel identified as ISDN primary access. An ISDN primary access provides twenty three 64-kbps B channels plus one 16-kbps D channel for a total capacity of approximately 1.5 megabits per second ("mbps"). CCITT envisions that the ISDN primary access can be used for communications between an ISDN local exchange and an ISDN Private Branch Exchange ("PBX").
Because the CCITT standard for the ISDN communication channel specifies the lowest three layers of the OSI model, the ISDN standard provides interfaces, both physical, e.g., the plug in a wall, and logical, e.g., electrical signals passing through the plug. In achieving this result, the ISDN standard specifies several different physical. interfaces, the most widespread of which is called the S interface. The S interface of the ISDN standard specifies the interface between Terminal Equipment ("TE"), e.g., a telephone, and a Network Termination ("NT") of the ISDN communication channel.
In North America, the S interface is the four wires usually found in a home telephone installation. In this interface, two of the four wires transmit data from the Network Termination to the Terminal Equipment, and two wires transmit data back from the TE to the NT. That is, the NT uses one pair of the four wires to transmit the combined B1, B2 and D channels of ISDN basic access to the TE, while the TE simultaneously transmits a different combined B1, B2 and D channels back to the NT on a different pair of the four wires.
While ISDN basic access was originally intended to provide voice and slow speed data communication services such as those identified above, over the years developments in digital signal processing and compression techniques have advanced technology to the extent that compressed video data may now be transmitted using ISDN basic access. These techniques have progressed to such an extent that there now exist several alternative video data compression techniques such as the CCITT H.261 picture phone standard, the Joint Photographic Experts Group ("JPEG") standard, and the Moving Picture Experts Group ("MPEG") standard.
U.S. Pat. No. 5,027,400, that issued Jun. 25, 1991, on an application filed in the names of Toru Baji et al. ("the Baji et al. patent"), discloses a multimedia bidirectional broadcasting system that distributes motion picture data using a broadband ISDN communication channel. In the system depicted in FIG. 3 of the Baji et al. patent, a motion picture program data base is maintained at a broadcasting station for transmission over broadband ISDN communication channels in response to requests received at the broadcasting station from subscriber systems. In the broadcasting station disclosed in the Baji et al. patent, an image encoder compresses a video signal prior to its transmission over the broadband ISDN communication channel to the subscriber system. The subscriber system includes a decoder for decoding the compressed video data and a television monitor for displaying them. Also included in the broadcasting station depicted in FIG. 3 is a cell disassembler that transfers control information received from the subscriber system over the ISDN communication channel to a main control unit of the broadcasting station.
In the broadcasting station depicted in FIGS. 1-3 of the Baji et al. patent, a subscriber system submits a reservation to the broadcasting station to access a program stored there. A group of video buffers, also depicted in FIGS. 1-3, permits the broadcasting system to simultaneously process data bases for a plurality of subscriber systems. A limitation of the broadcasting station disclosed in the Baji et al. patent occurs if the number of data bases available at the broadcasting station is insufficient for the number of subscriber systems requesting them. Under such circumstances, even though the subscriber system can communicate with the broadcasting system through an ISDN communication channel, the broadcasting system notifies the subscriber system attempting to make a reservation of how long it must wait before the data base will become available.
A playback control function, depicted in FIG. 15 of the Baji et al. patent, permits a subscriber system to control a program being transmitted from the broadcasting station, such as fast forwarding it, rewinding it, temporarily stopping it, or displaying it slowly. FIGS. 1-6 of the Baji et al. patent depicts a subscriber system that includes an image encoder for compressing a video signal from a video tape recorder, an optical disk or a real-time video camera prior to transmitting the compressed signal to the broadcasting station. At the broadcasting station, the compressed signal from the subscriber system apparently passes through the cell disassembler to be recorded in a video mail file from which other subscriber systems may retrieve it.
A limitation of the broadcasting system disclosed in the Baji et al. patent is that it lacks the ability to adapt compressed video data to the various different compression techniques such as CCITT H.261, MPEG or JPEG. The illustration of FIGS. 1-6 depicts the video mail file for storing compressed video data transmitted to the broadcasting system from subscriber systems. The text of the Baji et al. patent states that the broadcasting system includes an interface for accessing the video mail file. The illustration of FIGS. 1-6 shows that transmitted video mail file data passes directly from the video mail file to the broadcasting system's cell assembler, thus bypassing the broadcasting system's image encoder. Consequently, data stored in the video mail file of the broadcasting system can be viewed only on a subscriber system capable of decoding video data compressed according to the same standard, e.g., CCITT H.261, MPEG or JPEG, as that employed by the subscriber system in transmitting the compressed video data over the ISDN communication channel to the broadcasting system.
Furthermore, the broadcasting system disclosed in the Baji et al. patent cannot provide real-time communication between two subscriber systems. Using the broadcasting station disclosed in the Baji et al. patent, two subscriber systems can communicate only if one system first stores video data in the broadcasting systems video mail file, after which the other subscriber system must retrieve the stored video data.
Yet another limitation of the broadcasting system disclosed in the Baji et al. patent is that it possesses the capability of transmitting only compressed video data. In addition to various alternative video data compression techniques identified previously, there now also exist a variety of different standards for compressing audio data, such as the CCITT standards G.711 and G.722, that adapt audio data for transmission over an ISDN communication channel.